1. Field Of The Invention
The present invention relates to a process for separation of iron ions with associated heavy metals from zinc ions and lighter metals in concentrated acidic wastewater. More particularly, the present invention separates these metals at different steps by controlling the pH at each step.
2. Discussion Of Background References
There are many mining leachate collection ponds, and other mixed metal solutions that are considered as hazardous waste due to the presence of soluble metals. Typical ponds are in the western United States and contain zinc, iron, and other heavy metals. Due to environmental considerations for the receiving aquatic communities, the environmental regulatory agencies have been placing fairly low limits on these metals contained in wastewaters discharged to state and federal water ways. Therefore, many industrial plants as well as municipalities that have leachate evaporative catch basins installed in their natural surface water shed areas will have to remove the metal components from these impoundments before the waters can be discharged.
Historically, water treatment to clarify water has been undertaken by using aluminum and ferric salts or more recently polyacrylamides. In these processes, the operating zone has generally been in the neutral pH range, 6.5 to 7.5. Within this range, ferric and aluminum salts are within their lowest solubility range as hydrated hydroxides. However, within this range, other metallic ions such as zinc, magnesium, cadmium, copper, lead, etc. continue to have a high solubility level and remain in the purified liquid.
In the past, these levels of metals were acceptable in effluents being discharged from first generation treating facilities (pH adjustment/solids settling). With the development of environment concern and subsequent experimentation to reduce discharged metal levels, a second level of effluent treatment processes has been developed. In these schemes, ferric hydroxide is formed to act as the attaching medium to agglomerate and precipitate the particular materials formed. It has been noted that selenium and vanadium ions tend to be adsorbed on these iron flocs so that this method has an additional benefit of removing other metals of concern.
Several systems have been developed to address the metals removal problem. Examples of these systems include the Permutit-SULFEX.TM. and Durion-heavy metals waste treatment processes. These systems perform a single precipitation at a specific pH level. These systems generate a single highly hydrated, combined metal hydroxide/sulfide sludge that typically is landfilled. However, the resultant treated wastewaters from these processes still contain significant levels of heavy metals. In some cases, the single pH operating level allowed certain metals to precipitate and then partially redissolve before final precipitation and filtration. Alternatively, at the single pH level, the minimum solubility point of some of the metals present was not reached. As a result, a significant level of several metals remained in solution to be discharged in the wastewater.
U.S. Pat. No. 4,680,126 to Frankard, et al. discloses separation and recovery of reusable heavy metal hydroxides from metal finishing wastewaters. This reference discloses precipitating ferrous metal ions present while maintaining the non-ferrous metal ions in solution and isolating the precipitate. Then the reference discloses precipitating the non-ferrous metal ions from solution. However, this reference discloses a method for treating wastewaters which initially have an alkaline pH.
A need exists for a commercial process for treating acidic wastewaters such as mining leachate.